Some inkjet printers perform duplex printing of a continuous media web, such as an elongated roll of paper, using a single pass with a single print zone that includes a single array of printheads. A media transport system including a series of rollers that moves the media web through the printer in a process direction. In the single-pass duplex configuration, the media transport moves the media web through the print zone for first-side printing by only a first group of printheads in the print zone. The media transport subsequently moves the media web through an inverter that flips the print medium to present the reverse surface for printing. The media transport then moves the inverted media web through the print zone a second time past a second group of printheads in the print zone for second-side printing. The first group of printheads and the second group of printheads are offset from each other in the cross-process direction with sufficient space to accommodate the first side and the second side of the continuous media web concurrently. Thus, the first and second groups of printheads operate concurrently to print on different portions of the first and second sides of the media web, respectively.
In order to maintain high quality printed output, the printer performs process direction registration of the multiple printheads in print zone. The process direction registration ensures that ink drops from different printheads land on predetermined locations of the media web as the media web moves past the printheads in the process direction. For example, in a multi-color configuration printheads that eject inks of two different colors are arranged at different locations along the media path. When the printheads are properly registered in the process direction, the relative timing of the operation of the inkjets from each printhead ensure that ink drops land on the predetermined locations of the media web as the media web passes both printheads at different times along the media path. Proper process direction registration enables the accurate reproduction of a wide range of colors using a smaller number of ink colors, such as cyan, magenta, yellow, and black (CMYK) inks. Errors in the process direction registration can, however, result in inaccurate color reproduction and other reductions in printed image quality.
In an existing process direction registration technique, all of the printheads in the print zone eject ink drops to form a printed test pattern on the media web, and an electronic controller adjusts the timing of the printheads relative to a reference printhead based on image data generated from the printed test pattern. As described above, the controller adjusts the relative timing of the printheads so that ink drops from different printheads arranged along the process direction land on the correct location of the print medium. In a single-pass duplex configuration, however, the two different sides of a single media web effectively act as two different print media. Existing methods require adjustments to the media path to ensure that the first side and the second side of the media web remain aligned with inter-document zones on both the first side and the second side of the media web passing through the print zone in tandem. The inter-document zones are blank regions of the media web between adjacent printed pages where the test patterns are printed without interfering with pages printed during a print job. The adjustment to the media path frequently requires manual intervention from an operator, which can reduce efficiency of operating the printer. Consequently, improvements to process direction registration techniques in inkjet printers that reduce or eliminate the need to align the first and second sides of the media web during a print job would be beneficial.